Demonstration of Electromagnetic Shielding Principles · 2016-01-13 · From A Text Book (Henry Ott...

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Scott Piper – General Motors Electromagnetic Shielding
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Transcript of Demonstration of Electromagnetic Shielding Principles · 2016-01-13 · From A Text Book (Henry Ott...

  • Scott Piper – General Motors

    Electromagnetic Shielding

  • Dipole Antenna – 160mm Long

  • Electric Field Radiation Pattern

    Farfield – 377ΩWave Impedance

    Nearfield

  • Electric and Magnetic Fields evaluated

    along blue line

  • Electric Field vs. Distance from

    Antenna

  • Magnetic Field vs. Distance from

    Antenna

  • Electric and Magnetic Fields vs.

    Distance from Antenna

  • Wave Impedance

    𝑧(Ω) =𝐸 𝐹𝑖𝑒𝑙𝑑 (

    𝑉𝑚)

    𝐻 𝐹𝑖𝑒𝑙𝑑 (𝐴𝑚)

  • Wave Impedance vs. Distance

  • Wavelength / 6 point

    500 mm – 252.07Ω

  • Linear X Axis

    500 mm – 252.07Ω

  • Electric Field Radiation Pattern

    Farfield – 377ΩWave Impedance

    Nearfield

  • Wave Impedance

    Electromagnetic waves can originate from an electric field or

    magnetic field source

    Near the source, electric or magnetic field can dominate

    Far from the source, the ratio between electric and magnetic

    fields is 377

    Why do we care?

    EMC shielding strategies are different depending on which

    fields (electric or magnetic) are of concern

  • Static Shielding Examples

  • Static Electric Field Shielding

  • Setup

    Block Charged to

    100 Volts

    Block Set to 0 Volts

  • Electric Field Vectors

  • Electric Field Magnitude Between

    Blocks

    We are interested in shielding the 0V block (left) from the electric field generated by the block on the right

  • Shield Added

    Metal Sheet

    +100 Volts

    0 Volts

  • Electric Field Shielding Performance

    With Shield Without Shield

  • Representation

    - +

  • Electric Field Shielding Performance

    Shield at 0 Volts Shield Floating

  • Static Magnetic Field Shielding

  • Setup

    Bar Magnet

    Victim (copper)

    Magnetic Field

  • Adjusted Scale

    Magnetic Field

    at Victim

  • Shield added

    Magnet

    Victim

    Aluminum Shield

  • Shielded Results

    With Shield

    Without Shield

  • From A Text Book (Henry Ott

    Electromagnetic Compatibility Engineering)

    Static magnetic fields can’t be stopped, but can be redirected by providing a low-reluctance path

    This path usually involves a material with a relative permeability (µr) greater than 1

  • Steel Shield Results

    With Shield

    Without Shield

  • Static Shielding

    Electric fields need a shield at the same potential as the

    victim circuit

    Magnetic fields need a shield with high permeability (µ)

  • Scott Piper – General Motors

    Acknowledgement to Jim Teune and Bogdan Adamczyk of Gentex Corporation and Grand Valley State University

    Demonstration of Electromagnetic

    Shielding Principles

  • Low Frequency Magnetic Field

    Shielding

  • 3D Model Setup

    8 mil thick

    Iron Box

    1.4 mil thick

    Copper plate

  • Cross Section of Box

  • Model Excitation

    Plane Wave (farfield)

    1 V/m in Amplitude

    From 100 kHz – 10 MHz

  • Time Animation of H Field Showing Box Cross Section

  • New Model

    15 mils thick iron sides

    Electrically thick

    (Fields cannot penetrate)

    Copper sheet

    Considered thick for simplicity

    (Fields cannot penetrate)

    Nickel Silver

    8 mils thick

    Excitation is the

    same as in the

    previous example

  • Fields Observed under Box Lid

  • Magnetic Field – 10 MHz

  • Magnetic Field – 7 MHz

  • Magnetic Field – 5 MHz

  • Magnetic Field – 2 MHz

  • Magnetic Field – 1 MHz

  • Magnetic Field – 700 kHz

  • Magnetic Field – 500 kHz

  • Magnetic Field – 200 kHz

  • Magnetic Field – 100 kHz

  • Zoomed on Cross Section of Lid

  • Magnetic Field– 10 MHz

    Lid

    Inside Box

    Outside Box

  • Magnetic Field– 7 MHz

  • Magnetic Field– 5 MHz

  • Magnetic Field– 2 MHz

  • Magnetic Field– 1 MHz

  • Magnetic Field– 700 kHz

  • Magnetic Field– 500 kHz

  • Magnetic Field– 200 kHz

  • Magnetic Field– 100 kHz

  • Magnetic Field Shielding

    Magnetic field attenuates as it passes through a metallic

    medium – this is called Absorption Loss

    Absorption loss is greater as the frequency of the magnetic

    field increases

  • Absorption Loss

    The rate of absorption loss depends largely on the shielding

    material used

    Skin depth is the dimension at which the current falls to

    1/e of the current found on the surface (was once measured

    in Nepers) – (this is about 1/3 or 9 dB)

    Skin depth depends on frequency (f), permeability (µ),

    and conductivity of the material (s)Skin Depth in inches

    (source: Henry Ott

    Electromagnetic

    Compatibility Engineering)

  • Skin Depth of Various Materials

    Skin Depth in inches

    (source: Henry Ott

    Electromagnetic

    Compatibility Engineering)

    Material µr srd (mils)

    100 kHz

    d (mils)

    10 MHz

    Steel 1000 0.1 0.8 0.1

    Copper 1 1 8 0.8

    Phosphor Bronze 1 0.15 21 2.1

    Nickel Silver 1 0.06 33 3

  • Low Frequency Electric Field

    Shielding

  • Fields Observed under Box Lid

  • Electric Field – 100 kHz

  • Electric Field – 200 kHz

  • Electric Field – 500 kHz

  • Electric Field – 700 kHz

  • Electric Field – 1 MHz

  • Electric Field – 2 MHz

  • Electric Field – 5 MHz

  • Electric Field – 7 MHz

  • Electric Field – 10 MHz

  • Fields Viewed from Cross Section

    through Box

  • Electric Field – 100 kHz

  • Electric Field – 200 kHz

  • Electric Field – 500 kHz

  • Electric Field – 700 kHz

  • Electric Field – 1 MHz

  • Electric Field – 2 MHz

  • Electric Field – 5 MHz

  • Electric Field – 7 MHz

  • Electric Field – 10 MHz

  • Zoomed on Cross Section of Lid

  • Electric Field– 100 kHz

  • Electric Field– 10 MHz

  • Electric Field Shielding

    A sudden change of impedance causes electric fields to

    reflect off the surface of a metal shield – this is called

    Reflection Loss

    Reflection loss is somewhat independent of frequency

    As frequency increases, typically the shielding effectiveness of

    a shield decreases due to apertures in the shield

    𝑅 = 20 log𝑧𝑤4 𝑧𝑠

    Wave Impedance

    Shield Impedance

  • Wave Impedance vs. Distance

    The Characteristic Impedance of Copper is 3.68 x 10-7 Ω(3.68 mΩ at 100 MHz - Big difference from 10kΩ)

    As frequency goes up (and distance gets further), this gap narrows

    𝑓

  • Shielding Effectiveness

    Absorption Loss + Reflection Loss will give (in most cases)

    shielding effectiveness

    Shielding effectiveness is given in decibels and is a ratio of the

    signal with the shield present vs. the shield absent

    When communicating values of shielding effectiveness, it’s

    important to indicate if this is for electric or magnetic field

    shielding.

    Transmitted power shielding effectiveness is also used

  • Demonstration Setup

  • -130

    -120

    -110

    -100

    -90

    -80

    -70

    -60

    0 200 400 600 800 1000 1200 1400 1600 1800 2000

    Spe

    ctr

    um

    An

    aly

    zer

    Me

    asu

    rem

    en

    t (d

    Bm

    )

    Frequency (kHz)

    Power Supply Shielding of Various 8 Mil Thick Materials

    No Shield

    Nickel Silver

    Phosphor Bronze

    Copper Tape

    Cold Rolled Steel

  • -130

    -120

    -110

    -100

    -90

    -80

    -70

    -60

    0 200 400 600 800 1000 1200 1400 1600 1800 2000

    Spe

    ctr

    um

    An

    aly

    zer

    Me

    asu

    rem

    en

    t (d

    Bm

    )

    Frequency (kHz)

    Power Supply Shielding of Various 8 Mil Thick Materials(Envelope)

    No Shield

    Nickel Silver

    Phosphor Bronze

    Copper Tape

    Cold Rolled Steel

  • Skin Depth of Various Materials

    Skin Depth in inches

    (source: Henry Ott

    Electromagnetic

    Compatibility Engineering)

    Material µr srd (mils)

    100 kHz

    d (mils)

    10 MHz

    Steel 1000 0.1 0.8 0.1

    Copper 1 1 8 0.8

    Phosphor Bronze 1 0.15 21 2.1

    Nickel Silver 1 0.06 33 3

  • -130

    -125

    -120

    -115

    -110

    -105

    -100

    -95

    -90

    -85

    -80

    0 200 400 600 800 1000 1200 1400 1600 1800 2000

    Spe

    ctr

    um

    An

    aly

    zer

    Me

    asu

    rem

    en

    t (d

    Bm

    )

    Frequency (kHz)

    Power Supply Shielding of Phosphor Bronze of Different Thickness

    8 mil

    15 mil

  • Shielding Materials

    Low frequency magnetic field shielding requires a material

    with a thin skin depth.

    At higher frequencies, skin depth of most materials becomes

    thin enough creating effective shields

    Increasing shield thickness improves shielding until the

    material becomes thick enough.

  • Additional Thought about Magnetic

    Field Shielding

  • Setup

    2.5 cm thick wall

    Current Carrying Wire

    (10A)

  • Hypothetical Case There is a 5cm cube with a small amount of hollowed out

    material in the center

    There is a loop of current carrying wire at all frequencies surrounding the cube

    The cube is made of a particular material – the materials considered will be

    Vacuum (no cube)

    Aluminum

    Iron

    “Unobtainium” – has the same skin depth as iron except the relative permeability of the material is 1. but the relative conductivity is 688.

  • Magnetic Field Inside Box

    0.01 0.10 1.00 10.00 100.00 1000.00 2274.54Freq [kHz]

    0.00

    10.00

    20.00

    30.00

    40.00

    50.00

    53.30

    Y1

    [A

    _p

    er_

    me

    ter]

    Maxwell3DDesign1XY Plot 1

    Curve Info

    Aluminum BoxImportedPhase='0deg'

    Iron BoxImportedPhase='0deg'

    Unobtainium BoxImportedPhase='0deg'

    No BoxSetup1 : LastAdaptivePhase='0deg'

    No Box

    Al Box

    Iron/Unobtainium box

  • Iron vs. Unobtainium

    0.01 0.10 1.00 10.00 100.00 1000.00Freq [kHz]

    0.00E+000

    2.00E-009

    4.00E-009

    6.00E-009

    8.00E-009

    1.00E-008

    Y1

    [A

    _p

    er_

    me

    ter]

    Maxwell3DDesign1XY Plot 2Curve Info

    Iron BoxImportedPhase='0deg'

    Unobtanium BoxImportedPhase='0deg'

    Iron

    Unobtainium

    This suggests that permeability improves shielding beyond absorption loss below 100 Hz

  • Effect of Apertures on EM Shielding

  • Model Setup

  • Principle of Reciprocity

    Most shields are passive

    The ability of a shield to stop an exterior field (immunity) is

    equal to its ability to stop an interior source (emissions)

    When determining shielding effectiveness, either an

    emissions or immunity case can be studied – whichever is

    more convenient

    Typically, it’s easier for physical measurement to measure

    emissions

    Typically, it’s easier for simulations to measure immunity

  • Model Excitation

    X Direction

  • Probe Locations

    Height is at PCB Outer Layer Level

  • Electric Field (X Plane Wave)

    at 162 MHz

    Break

    Holes

    Center

  • Shielding Effectiveness over Frequency

  • H field Scanner

  • Laboratory Measurement

    Showing H field Product Emissions at

    162 MHz

  • H Field Plot at 162 MHz

    X Plane Wave

  • Vent Holes Removed

  • H Field Plot at 162 MHz

    X Plane Wave

    Model with Vents Model Without Vents

  • H Field Laboratory Measurement

    At 162 MHz

    Measured With Vents Measured Without Vents

  • E Field Plot

    At 162 MHz

    With Vents

    Without Vents

  • Shield Apertures

    Electrically small apertures reduce electric field shielding

    when the source/receiver is close to the aperture

    This demonstration shows that apertures of a certain nature

    can impact electric field shielding but not magnetic field

    shielding

    E field With Vents E Field Without Vents

  • Slots

    9/18/2015

  • Simple Radiation Pattern

  • Simple Slot Graph

  • Slots Slots can be created in a shield through several ways

    If two metallic surfaces are touching but not pressed together, there is a poor connection between these two surfaces

    Seams

    Lids

    Gaskets

    Paints

    A shield connection should NOT be through a screw!

  • Base Line Shield

  • Applied EMI Wave

  • Real Shield without Slot Graph

    9/18/2015

  • Real Shield with Slot

  • Slot Broken in Two

  • Slot Broken in Four

  • Summary Graph

  • Simulation of a PCB shield with various

    lifted terminations:

  • PCB Shield Terminations

    Slot antennas are created between shield termination points

    Fewer shield terminations may be easier to install but may

    create resonant conditions at critical frequencies

    Shields reduce the fields at lower frequencies with a risk of

    creating resonance at higher frequencies

  • What Makes an Effective Shield?

    Static Electric Field

    ?

    Static Magnetic Field

    ?

    Low Frequency Electric Field

    ?

    Low Frequency Magnetic Field

    ?

    High Frequency Electromagnetic

    Field

    ?

  • What Makes an Effective Shield?

    Static Electric Field

    Same Potential as

    Victim Circuit

    Static Magnetic Field

    High Permeability

    Material

    Low Frequency Electric Field

    High Conductivity

    Material

    Low Frequency Magnetic Field

    Thick Material

    (compared with skin

    depth)

    High Frequency Electromagnetic

    Field

    Small Apertures (compared

    to wavelength)